For example, a color image forming apparatus using an electrophotographic image forming method uniformly charges the surfaces of respective photosensitive bodies (respective scan objects) corresponding to a plurality of colors and then scans the respective photosensitive body surfaces with respective light beams so as to form respective electrostatic latent images on the respective photosensitive body surfaces. The color image forming apparatus develops the electrostatic latent images on the respective photosensitive body surfaces using respective colors of toner to form toner images in the respective colors on the respective photosensitive body surfaces. The color image forming apparatus superimposes and transfers the toner images in the respective colors from the respective photosensitive bodies to an intermediate transfer body so as to form a color toner image on the intermediate transfer body, and then transfers this color toner image from the intermediate transfer body to a recording paper sheet.
The respective photosensitive bodies are scanned with the respective light beams by a light scanning device. Typically, four colors, which are black, cyan, magenta, and yellow, of toner are used. Accordingly, it is necessary to scan four photosensitive bodies using at least four light beams, and four light-emitting elements for emitting the four light beams need to be used.
Nowadays, there is a need for downsizing and thinning of the image forming apparatus, and a downsized and thinned light scanning device becomes necessary. Accordingly, there is proposed a light scanning device with the following configuration. A polygonal mirror (deflecting section) is arranged approximately in the center of the light scanning device. Two optical systems are arranged symmetrical with respect to the polygonal mirror at the center. Respective light beams emitted from the respective light-emitting elements are reflected by the polygonal mirror so as to be divided into the respective optical systems. The respective optical systems cause the respective light beams to enter the respective photosensitive bodies.
On the other hand, a BD sensor is disposed to detect a light beam deflected by the polygonal mirror, and the scanning timing on the photosensitive body using the light beam is set based on the detection timing of the light beam using the BD sensor. In short, the scanning timing on the photosensitive body using the light beam is synchronized with the detection timing of the light beam using the BD sensor.
Here, the light beam is reflected by the polygonal mirror to be repeatedly deflected in the range having an approximately fan shape. This range having an approximately fan shape includes the scanning angle range of the light beam that scans a scan object. The BD sensor is often disposed outside the scanning angle range of the light beam. For example, in Patent Literatures 1 and 2, a BD sensor is arranged outside the scanning angle range of the light beam, and the light beam deflected by a polygonal mirror enters the BD sensor.
In Patent Literatures 3 and 4, a detecting mirror and a BD sensor are arranged outside the scanning angle range of the light beam, and the light beam deflected by a polygonal mirror is reflected by the detecting mirror such that the light beam enters the BD sensor.